Electron discharge device with arrangement for replenishing emissive material on a smooth cathode surface



Sept. 16, 1969 s. P. NEWBERRY 3,467,878

ELECTRON DISCHARGE DEVICE WITH ARRANGEMENT FOR REPLENISHING EMISSIVE MATERIAL ON A SMOOTH CATHODE SURFACE Filed June 27, 1966 u manta/Ina:

[)7 1/617 to)": Sterflhg P New/berry by d? H/Is Attorney:

United States Patent US. Cl. 313346 8 Claims ABSTRACT OF THE DISCLOSURE An electron discharge device includes an anode which has a reservoir for continuously providing emissive material to a point cathode as emitter material is lost by the cathode during operation. The anode is provided with a heater to evaporate the emissive material as required, the potential difference between the cathode and anode causing the evaporated material to be deposited on the cathode.

The present invention relates to an electron discharge device having an improved dispenser cathode. More specifically, it relates to a dispenser cathode with an external supply of emissive material from which the emitter surface is continually replenished during operation of the cathode.

Electron discharge devices, such as cathode ray tubes, are commonly used in a variety of applications. Typical of these applications are oscilloscopes, electron microscopes, flying spot scanning devices, and electron beam recorders. Such devices require the emission of a continuous stream of electrons preferably concentrated in a narrow path. Thermionic emission or heat-induced discharge of electrons from an emissive material disposed on an emitter surface is commonly used to provide this stream of electrons. The tendency of the emissive material to be lost from the emitter surface, such as by evaporation, is often a significant factor in shortening the operable life of these devices.

The development of the dispenser cathode was an attempt to counteract this tendency and to prolong the life of electron discharge devices. The dispenser cathode includes an electron emitting element with a reservoir of emissive material from which that on the emitter surface is continually replenished. In its most common form, the dispenser cathode consists of a supply of emissive material located adjacent or subjacent to the emitter surface. Upon the application of heat, the emissive material migrates to the emitter surface and spreads over the surface in a thin layer on the order of one molecule thick. This process continues during operation of the device to replenish any emissive material lost from the emitter surface.

In the customary form of dispenser cathode, where the dispenser supply is an integral part of the cathode, three temperature sensitive processes are involved, namely, thermionic emission temperature, emission material, volume diffusion temperature and emission material reduction temperature for decomposition of the chemically bonded emission material in the dispenser reservoir. In general these processes optimize at different temperatures and while the diffusion temperature can be modified, with great difficulty, toward either of the remaining temperatures by pore size and distribtuion control, the reduction temperature and emission temperature cannot in general be made to coincide.

Dispenser cathodes of the customary type have a number of other undesirable features. For instance, the proximity of the emissive material reservoir with the emitter surface imposes severe limitations on the size and physical "ice configuration of the cathode structure. In addition, the necessary physical contact between the supply of emissive material and the emitter surface renders it very difficult to maintain the emitter supply and emitter surface at their optimum temperatures, unless these happen to coincide, since heat may flow freely from one to the other. A further problem arises due to the formation or deposition of impurities on the emitter surface. Such impurites may inhibit the activity of the emissive layer by interfering with the emission process. The most serious problem arises, however, from the fact that the emissive material reservoir is usually located beneath the cathode emitter surface, which must necessarily be porous to permit migration of the material from the reservoir to the emitter surface. On a microscopic scale, the porous surface is seen to be a combination of discontinuities and projections. This irregular surface causes serious degradation of the geometrical perfection of the electron source so that it is not possible to direct the electrons into a narrow wellcoordinated beam. The increasing need for physically smaller electron sources is making increasing demands for smoother cathode surfaces. In addition the combination of projections and pits on the surface gives rise to patchy emission so that only about 30% of the cathode area is used effectively.

In order to provide for emitter surfaces which can be kept at optimum emission temperatures, dispenser cathodes have also been proposed in which the supply of emissive material is physically separated from the cathode surface. In these cathodes the supply of emissive material is placed in a heated cup below the cathode surface, i.e., the side opposite that from which electrons are emitted. Thus, these cathodes also require a porous emitter surface, since the emissive material must migrate from the reservoir located beneath the emitter surface. Therefore, the problems of an ineflicient emitter surface, as the result of discontinuities in the surface, and limitations both as to size and physical configuration of the cathode are inherent in this proposed type of dispenser cathode, also.

Other forms of dispenser cathodes have been proposed in which a thermally sensitive compound is made to completely fill the entire vacuum structure by gaseous diffusion and then made to decompose at the cathode surface by heating the cathode during the diffusion process. This results in contamination of the entire structure of the device and also requires cyclic shutdown of the equipment for cathode regeneration.

Accordingly, it is an object of my invention to provide an electron discharge device with a dispenser cathode free of the inherent deficiencies and disadvantages of known dispenser cathodes as previously described.

Another object is to provide an electron discharge device with an externally supplied dispenser cathode.

It is a further object of this invention to provide an electron discharge device with a dispenser cathode having an efficient emitter surface.

It is also an objective to provide a cathode with a smooth surface so that a favorable source geometry may be established for optimum focusing of the electrons derived from the cathode.

Another objective is to provide an electron discharge device with a disepnser cathode having a reservoir of emissive material located such that limitations on cathode temperature and on the size and physical configuration of the device are minimized.

Another object is to provide an electron discharge device, having a dispenser cathode, which is simple and inexpensive to fabricate.

Briefly, these and other objects are met in accordance with my invention by providing a supply of emissive material on the anode, or an adjacent structure substantially at anode potential, opposite the cathode of an electron discharge device. During operation of the device, the emissive material is induced to transfer from the anode to the emitter surface of the cathode. This may be accomplished by using an emissive material which vaporizes to the positively ionized state, including, for example, the alkaline earth metals and the alkali metals, such as cesium. An anode heater may be used to induce reduction and evaporation of this material. Once ionized, the emissive material is attracted by the negative potential of the electrical field existing between the cathode and anode and is thus induced to transfer to the electron source at the cathode surface.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic illustration of an electron discharge device in which the preferred embodiment of the present invention is incorporated, and

FIGURE 2 is a schematic illustration of a cathode ray tube in which the electron discharge device of FIGURE 1 is incorporated. 7

Referring more specifically to FIGURE 1, there is shown schematically a cathode 1, in cross-section, having a smooth emitter surface 2, and a cathode heater 3. Opposite the cathode is an anode 4, also in cross-section, a reservoir of emissive material 5, which evaporates to the positively ionized state, and an anode heater 6. The anode is also provided with an opening 7 to permit passage, through the anode, of the electrons emitted by the cathode. As shown, the emissive material reservoir may consist of a coating of emissive material on the surface of the anode facing the cathode. Alternatively, the emissive material may be mechanically secured or impregnated on the anode surface or it may be integrally associated with the anode material. In the latter embodiment, a particulate form of the emissive material is dispersed through the material of which the anode is formed.

The anode, together with its associated emissive material reservoir, as disclosed in the present invention, may be made in a number of ways. For example, a layer of the emissive material may be applied to the anode surface by painting or spraying from a molten form or in solution. Alternatively, particles of the emissive material may be incorporated into a mass of material from which the anode is to be formed thereby providing, when the anode is formed, an anode including the emissive material dispersed throughout. Still another method by which the emissive material may be associated with the anode is by impregnating, painting, spraying or incorporating into the anode material in some other way, a heat decomposible compound containing the emissive material. As examples of the latter type of compound, nitrates and carbonates of materials which vaporize to the positively ionized state may be used. Another similar material which may be used is cesium iodide. All of these materials will decompose under the influence of heat to leave the emissive material disposed either in or on the anode.

Referring now to FIGURE 2 there is shown a cathode ray tube embodying the electron discharge device illustrated in FIGURE 1. The cathode ray tube illustrated in FIGURE 2 further includes a second anode 8 to prevent dispersion of the electron beam, and horizontal and vertical electrostatic guides or deflectors 9 and 10, shown in isosymmetric view, to direct the electron beam onto any specific point of the tube screen or beam target 11. An enclosure or housing 12 is also provided for the cathode ray tube and a power feed 13 to the cathode, including an anode-biasing potentiometer.

With respect to the operation of the device shown in FIGURE 1, a small amount of emission material is first deposited upon the emitter surface of the cathode structure by simultaneous application of heat to the anode and an electric field between anode and cathode. Upon raising the temperature of the cathode, electron emission will occur. The positively biased anode causes the electrons emanating from the cathode to be projected through the opening in the anode. By sensing the current flow from the cathode in the power feed to the cathode 13, the temperature of the anode can be controlled continuously or intermittently to supply additional emission material to the cathode as needed without interrupting or interfering with the cathode action. This process may be controlled manually as well as automatically. As the anode heater is energized, either periodically or continuously, depending on the necessity for replenishing the supply of emissive material on the cathode emitter surface,

r new emissive material is evaporated from the anode. The

vaporized emissive material, which becomes positively charged upon evaporation, is attracted to the negatively charged cathode and is deposited thereon, thereby replenishing the supply of emissive material on the cathode emitter surface.

In the preferred embodiment of the present invention, which is illustrated in FIGURE 1, in addition to using an emitter surface which is smooth and highly efiicient with respect to its emission characteristic, a particular cathode geometrical configuration, more specifically a point cathode, is used. The point cathode provides a very thin stream of emanating electrons and a very concentrated negative field for attracting'the positively ionized vaporized emissive material. This results from the fact that in a point cathode, electron emanation takes place primarily from that part of the cathode in closest proximity to the anode, which is the point of the point cathode.

Improvements both in the efficiency of these devices and in the service life may be expected as a result of the optimization of the cathode emitter surface and the operating temperatures of the emitter surface and the migrative emissive material reservoir. More specifically, emitter surfaces of any desired degree of smoothness may be used without regard to the permeability or transferabiliy of the surface to migrative emissive material. Further, the emitter surface may be maintained at the optimum temperature for electron emanation without regard to the emissive material supply which is physically separate in this invention. Similarly, the emissive material supply may be maintained at an optimum temperature to permit its transfer to the emitter surface at a desired rate. As a result, the device may be operated in a highly efficient manner while replenishing the emissive material on the emitter surface at a rate consistent with the expected or measured losses from that surface. In this manner, the service life of the device may be effectively prolonged.

By way of pointing out a further advantage of my invention, it should be noted that replenishment of the emissive material on the cathode surface takes place from the outer or exposed portion of that surface. In effect, this extends the emitter surface over any impurities or oxides which may be deposited on the emitter surface and which would otherwise have a detrimental effect on the emissivity of the surface.

The service life of electron discharge devices produced in accordance with the preferred embodiment of this invention is extended even further by the probability that loss of emmisive material from the cathode emitter surface is lessened due to the tendency of the emissive material, which vaporizes to the positively ionized state, to be attracted by the electrical field back to the electron source at the cathode emitter surface.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without departing from the true spirit and scope and the invention. Therefore, the appended claims are intended to cover all such equivalent variations which come within the true spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electron discharge device having a cathode, a cathode heater, an anode, an anode heater, a supply, physically associated with the anode, of emissive material which evaporates to the positively ionized state and means impressing a potential diiference between said cathode and anode whereby, when said anode heater is energized, emissive material evaporated from said supply is attracted to and deposited on said cathode to replenish emissive material removed from said cathode during operation of the device.

2. An electron discharge device, as in claim 1, wherein the cathode is a point cathode.

3. An electron discharge device, as in claim 1, wherein the cathode has an emitter surface which is highly efiicient with respect to its emission properties.

4. An electron discharge device, as in claim 1, wherein the emissive material comprises an alkaline earth metal. 5. An electron discharge device, as in claim 1, wherein the emissive material comprises an alkali metal.

6. An electron discharge device, as in claim 1, wherein the emissive material is cesium.

7. An electron discharge device, as in claim 1, wherein the emissive material forms a layer on a surface of the anode facing the cathode.

8. An electron discharge device, as in claim 1, wherein the emissive material is integrally associated with the material of which the anode is formed.

References Cited UNITED STATES PATENTS JOHN W. HACKERT, Primary Examiner ANDREW J. JAMES, Assistant Examiner US. Cl. X.R. 

